1. Field of the Invention
The present invention relates generally to electromagnetic interference (EMI) protective measures and, more particularly, EMI protective measures for printed circuit boards.
2. Related Art
Most countries in the world have regulations that limit the amount of electromagnetic emissions that electromagnetic equipment may produce. Electromagnetic emissions are the unwanted byproduct of high-frequency electronic signals necessary, for example, to operate an electronic microprocessor or other logic circuitry. The electromagnetic interference (EMI) that results is a problem when it interferes with licensed communications, such as television, radio, air communications and navigation, safety and emergency radios, etc. This type of interference has also been known as radio-frequency interference (RFI). See CFR 47 part 15 and ANSI publication C63.4-1992 for regulations in the United States, or CISPR publication 11 or 22 for international regulations. Also, “Noise Reduction Techniques in Electronic Systems” by Henry W. Ott, serves as an excellent reference on the current art for the control of EMI, and the broader topic known as electromagnetic compatibility (EMC).
To meet EMI regulations, most electronic equipment currently employs a combination of two approaches commonly referred to as ‘source suppression” and “containment.” Source suppression attempts to design components and subsystems such that only essential signals are present in signal interconnections, and that all non-essential radio frequency (RF) energy is either not generated or attenuated before it leaves the component subsystem. Containment attempts to place a barrier around the assembled components, subsystems, interconnections, etc., so that any unwanted electromagnetic energy remains within the boundaries of the product, where it is dissipated harmlessly.
This latter approach, containment, is based on a principle first identified by Michael Faraday (1791-1867), that a perfectly conducting box that completely encloses a source of electromagnetic emissions would prevent those emissions from leaving its boundaries. This principle is employed in conventional shielded cables as well as in shielded enclosures. Conventional shielded enclosures usually consist of a metal box or cabinet that encloses the equipment. The metal box is often supplemented with additional features as necessary in an attempt to keep RF energy from escaping via the power cord and other interconnecting cables. The metal shield may be structural, for example, the product enclosure itself. For example, a product enclosure might consist of a plastic structure with a conductive coating on the surface. This approach is commonly implemented in, for example, cell phones. More commonly, the metal shield is implemented as a metal “cage” inside the product enclosure since the EMI suppression required for the entire product or system requires that only a portion of the product be shielded. Such metallic cages are placed around components, or around subsystems when additional EMI reduction is required.
There are numerous drawbacks to the use of such metallic boxes. These drawbacks are primarily related to the lack of shielding effectiveness provided by conventional metallic boxes. For example, the metallic box creates a stagnant buffer of insulating air around the component causing the temperature of the component to increase. In such products, the enclosure typically includes cooling holes and fans to circulate air around the metallic box to dissipate the heat. In addition, electromagnetic energy often escapes the shield at gaps between the shield and the printed circuit board. Electrical gaskets and spring clips have been developed to minimize such leakage. Unfortunately, they increase the cost and complexity of the printed circuit board, and have limited success. In addition, leakage occurs because the cables and wires penetrating the shield are not properly bonded or filtered as they exit the metallic box. Further drawbacks of metallic cages include the added cost and weight to the printed circuit board assembly, as well as the limitations such metallic boxes place on the package design.